In order to handle increasing communication traffic in recent years, a further increase in communication (transmission) capacity is desired. In optical communication systems using a single mode fiber (SMF) used for conventional optical communication, however, a limit to increasing capacity is predicted. As a technique to overcome the limit, active research and development of space division multiplexing have been carried out.
Optical fibers for space division multiplexing include few-mode fibers (FMFs), where a capacity is increased by propagating a plurality of modes within one core and placing signals on each of the modes, and multicore fibers (MCFs) where a capacity is increased by placing signals on each of a plurality of cores (see Patent Documents 1 to 6 for MCFs).
Basically, the MCFs are roughly classified into two types of uncoupled MCFs where each core individually transmits information and coupled multicore fibers (C-MCFs) where (modes of) respective cores are coupled and thereby form a super-mode and information is transmitted by each super-mode (see Non-Patent Documents 1 to 3 for C-MCFs).
Especially in few-mode transmission, mitigating a differential mode group delay (DMGD) among propagation modes is important for shortening calculation time of signal processing such as multiple-input and multiple-output (MIMO). In the C-MCF, examination on enhancing coupling of super-modes in order to mitigate a DMGD has been carried out and has recently been attracting attention (see Non-Patent Documents 2 and 3).